Mr Putin, who steps down as Kremlin leader next month, told nearly 600 party delegates on Tuesday "I accept the invitation of the party".

Mr Putin is due to hand over the presidency to his long-standing aide Dmitry Medvedev. Mr Putin confirmed that he would become prime minister. Correspondents say the party job will give Mr Putin an additional power base.

United Russia controls two-thirds of the seats in parliament, and exerts great influence through its administrative and financial means. The party congress in Moscow was shown live on state-run television.

Mr Medvedev declined to join United Russia, saying such a move would be "premature". He is due to be sworn in as president on 7 May. He won the 2 March election by a landslide.Referring to Mr Medvedev's decision, Mr Putin said "I do not believe it is sensible for a head of state, wherever his political affections are, to lead a party. Here I fully agree with Dmitry Anatolyevich Medvedev".

"As for the chairman of the government (prime minister), a situation in which the head of the executive branch leads a party is a civilised and natural practice that is traditional for democratic states," Mr Putin told the congress.

"Co-ordinated work between the government and the parliamentary majority allows us to successfully resolve the tasks of developing the economy, enhancing the quality of healthcare and education, raising the income of the population and strengthening the country's defence."